A digital oscilloscope is a tool utilized by engineers to view signals in electronic circuitry. As circuits and signals get ever faster, it is beneficial to have digital oscilloscopes capable of digitizing, displaying and analyzing these faster signals. The capability of a digital oscilloscopes to digitize fast signals may be measured by its bandwidth and sample rate. The sample rate is the number of samples points taken of a waveform in a given amount of time and is inversely proportional to the sample period—the time between samples. If a sinusoidal frequency sweep is performed from DC up to higher frequencies, the bandwidth is the frequency at which the signal displayed on the digital oscilloscope screen is approximately 30% smaller than the input sine wave.
Since one of the uses of the digital oscilloscope is to design and analyze new electronic devices, high end digital oscilloscopes generally operate at speeds much higher than the present state of the art in electronics. These speeds may be achieved through the use of ever-faster sampling chips or the use of alternate methodologies to provide the desired bandwidth.
One such method involves triggering repeatedly on a periodic event. If an event is frequently, periodically repeating, the waveform at the time of the event can be repeatedly displayed on the screen. Data from multiple trigger events average together to provide a good view of the waveform. More particularly, the scope may repeatedly trigger on an event and acquire only a few points of the waveform (sometimes only one point of the waveform) on each trigger event. Scopes having this functionality are sometimes called “sampling scopes.” After repeated triggers, the points are reassembled according to the sampling algorithm to create a higher “effective” sample rate version of the waveform. Furthermore, the repeated trigger events permit averaging, which can be utilized to increase the signal-to-noise ratio (SNR) and therefore enable further bandwidth increases. However, such a sampling scope presupposes a repetitive input signal so that the representation of the waveform can be generated over many triggers.
This technique may be unsuitable where the signal that is to be analyzed is not repetitive. For instance, a non-repetitive event such as the cause of some failure in an electronic system. The trigger event may happen repeatedly but the signal around the trigger event may be different. Therefore, it is desirable to achieve a high bandwidth and sample rate with only a single trigger event. Such digital oscilloscopes are sometimes called real-time scopes, and acquisitions taken utilizing only a single trigger event are called single-shot acquisitions.
In real-time digital oscilloscope design, one method for improving sample rate is interleaving. This method utilizes multiple digitizing elements that sample the same waveform at different points in time such that the waveform resulting from combining the waveforms acquired on these multiple digitizers forms a high sample rate acquisition. Most high-end real-time digital oscilloscopes have very high sample rates achieved through the use of interleaving and most are capable of “oversampling” an input waveform.
Another technique is described in U.S. patent application Ser. No. 10/693,188, entitled “High Bandwidth Real-Time Oscilloscope,” filed Oct. 24, 2003 by Pupalaikis et al. and assigned to LeCroy Corporation (which is also the assignee of the instant application), now U.S. Pat. No. 7,058,548. Pupalaikis et al. describes a heterodyning technique wherein a low frequency channel acquires the low frequency content of the input signal and a high frequency channel acquires the frequency content of the input signal. This high frequency signal is mixed down from frequency band F→2*F to the range of 0→F so it “fits” into the bandwidth of the front end. It can be seen that twice the frequency content of the signal has been made to “fit” into the bandwidth of the scope. After processing, the high frequency content is mixed upward to its original frequency range and then combined with the low frequency content to generate an output waveform having approximately twice the bandwidth of that the scope would have been able to process otherwise.
In implementing this technique, the input signal may be filtered to separate the high frequency content from the low frequency content. Depending on the technique used to filter the input signal and the sharpness of the associated roll-off, the filter may affect the phase of the input signal non-uniformly. In particular, phase distortion may occur at the filter band edges.
Phase misalignment between overlapping portions of the low frequency content and the high frequency content, which is sometimes referred to as the crossover region, may cause those signals to cancel or partially cancel each other when combined, sometimes called destructive combination. The resulting aggregate response may accordingly have an undesirably attenuated magnitude response or a shifted phase response relative to the input waveform in one or more regions of interest.